When the integral function is placed inside a wait until next ms multiple is it reset at the beginning of each loop?

When the integral function x(t) is placed inside a wait until next ms multiple while loop is the Integrator reset to zero every time the loop executes?

Hi,
the function works on a discrete amount of data. Hence every call to it replaces the values used first time around.
I've attached an example in LabVIEW 7.0 that uses a pulse generation vi to allow the manipulation of the array sent into the integral object, and then it's easy to calculate what the integral of the data would be.
Hope that helps
Sacha Emery
National Instruments (UK)
// it takes almost no time to rate an answer
Attachments:
integral.vi ‏39 KB

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This question is really for the developers of LabVIEW, and I don't know if they are willing to divulge what might be considered "trade secrets". Anyway, I just thought there's no harm in asking.
I use the Wait Until Next ms Multiple vi and it works great. For example, if I set my program to output a character on the serial port every 20 millisecond (mS), and check the output on an oscilloscope, I see the output at 20 +/-2 mS (that is, 18 to 22 mS). That's quite good and is all that can be expected on a PC running a non-real-time OS such as Windows.
My question is this: Since the default Windows timer has a tick rate of 15.625 (1/64th of a second) (some PC's may default to 10 mS), how does LabVIEW attain roughly 1 mS accuracy? If the same thing is coded in any .NET framework program, such as C# or Visual Basic using any of the available timers (the System.Windows.Forms timer or the .System timer), the timer interval may be set in 1 mS increments, but if you time it you will see it actually has a granularity of 15.625 mS. So instead of getting 20 mS I get 2 ticks, or about 31 mS. So I can get an output every 15.6, 31, 47, 62.5, etc. but nothing in between.
The System.Diagnostics.Stopwatch timer can be use to time code with very high resolution, or to create a high-CPU usage delay loop, but I haven't been able to find an easy way of generating an interrupt (timer tick) at say 1 mS. There are Win API functions that can set the timer to 1 mS on XP and 0.5 mS on Windows 7 and 8 Microsoft, but I have not tried them. (They come with the caveat that another program or process can come along and reset your timer!) There is the "multi-media" timer (now the HPET or High Performance Event Timer"), but it is meant for sound and video at the driver/kernel level, and not for the UI level, as LabVIEW or C# are.
So how does the 'Wait Until Next ms Multiple work? It obviously is timer interrupt-driven because it yields to other processes (which is a primary reason for putting it in your overall While loop). And is it guaranteed on all recent OS versions (mine was timed on an dual core XP OS)?
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Ian,
Thanks for your reply. Yes, I'm sure LabVIEW uses the (default) Windows timer. And yes, 1 mS is not guaranteed due to the preemptive nature of Windows (and even "RTOSs" to varying degrees), which is why I see about plus or minus 2 mS.
Apparently the Windows timer can be set by API calls. See: http://www.lucashale.com/timer-resolution/. Here's a screen shot of his TimerResolution.exe on a Windows 7 PC:
Here it is on my Windows XP PC after I set it to "Maximum" (initially it was 15.625 mS):
Notice that it sets the Maximum to less than 1 mS, which is supposed to be the max, so there are some bugs. Plus the Default button does not reset it in XP, but does work on Windows 7 or 8. (I know this is not the place to "debug" non-LabVIEW applications!)
I'll bet LabVIEW sets it, too. The only caveat, as I said, is it looks like another application can change it, since the hardware timer is a "global" timer. I have not seen this issue in my LabVIEW applications, have you?
I guess I need to do some more digging to see the code to set the timer, but it looks like the developers of LabVIEW have it figured it out.
(FYI, I did notice that running my LabVIEW app (which gives about 2 mS resolution) or a C# app, which gives 15.625 mS resolution, does not affect what TimerResolution.exe reports, so I'm not sure if it's really working correctly. If I figure it out I'll post the results.)
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Question about the Wait Until Next ms Function

Hello,
I met a problem when I tried to use wait until next ms function to control the period of the loop.
I use Labview2009 now. I wrote a VI to sample the current values at 100Hz and then store the samples in a text file. The currents are sampled once in a loop and then wait for next 10ms to begin the next loop. After 360,000 loops (should be an hour), it will creat a new file to store the new samples in the following an hour.
I left the VI runs on CompactRio for the whole weekend. The result I got is shown in the picture below:
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When I checked the first three files (created from Fri 16:00 - Sat 16:00), I found that it didn't sample at 10Hz. It sampled at the frequency of about 20Hz (sometimes 18Hz, sometimes 19Hz).
I am confused by the problem a lot. What may cause it didn't work in the first 24 hours and worked after 24 hours? And How could I fix this problem?
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You need to show us some code!
This cannot happen in a single loop, but can easily happen if you have e.g. stacked loops or sequences containing multiple wait statements.
Also have a look at these old threads:
Synchronization problems during measurements (using while loop)
accurate timing between events
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When the integral function is placed inside a wait until next ms multiple is it reset at the beginning of each loop?

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